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Tiny zaps, big results

Speed read
  • High-speed lasers at the nanoscale yield remarkable material changes.
  • Computer simulations required to understand the transformations.
  • Laser irradiation poised to revolutionize pharmaceutical delivery.

When nanotechnology researchers make ‘small talk,’ it’s extremely small. They are interested in physical phenomena at one-billionth of a meter – a million times shorter than the length of an ant, or 100,000 times thinner than a human hair. Despite the size of their subject, the benefits to society are huge.

In fact, according to the National Nanotechnology Initiative, more than 800 everyday commercial products rely on nanoscale materials and processes for a variety of applications in medicine, energy, information technology, and many other areas.

Leonid Zhigilei.

One method of nanotechnology research involves the use of short laser pulses at minuscule fractions of a second to produce structural changes in thin, localized surface regions of materials like gold, silver, or silicon. Leonid Zhigilei of the University of Virginia says that what attracted him to this type of research is the ability of lasers to excite and change materials in ways not possible with any other technique.

For example, short-pulse laser processing can transform a surface from very water attractant to very water repellant, reducing friction, erosion, and contamination on items that need to be kept clean (e.g., roof tiles and skyscraper windows).

Because laser-induced processes are complex and happen so fast, experimentation cannot provide a detailed understanding of the structural transformations triggered by the rapid laser energy deposition, Zhigilei says.

“Our atomistic simulations, on the other hand, provide clear visual representations, or ‘atomic movies,’" adds Chengping Wu, a member of Zhigilei’s computational materials research group. “They are well-suited to reveal the relationships between the properties of laser-treated regions of the targets and the underlying microscopic mechanisms of laser-induced target modification.”

Zhigilei’s group uses the Darter supercomputer at the US National Institute for Computational Sciences (NICS) and Stampede at the Texas Advanced Computing Center (TACC).  High-performance computing has played a crucial role in many of their projects because the systems they simulate can consist of up to a billion atoms.

“Unlike in real experiments,” Zhigilei explains, “the analysis of non-equilibrium processes in molecular dynamics can be performed with unlimited atomic-level resolution, providing complete information of the phenomena of interest.”

The US National Science Foundation’s (NSF) eXtreme Science and Engineering Discovery Environment (XSEDE) made possible Zhigilei’s compute allocations on Darter and Stampede. XSEDE is a single virtual system that scientists can use to interactively share computing resources, data, and expertise. People around the world use these resources and services — things like supercomputers, data collections, and new tools — to improve our planet.

<b>Laser tag.</b> An illustration of some of the results of large-scale atomistic computer simulations of laser-induced modification of silver targets. The simulations make predictions about the surface and explain the experimental observation of surface swelling. Courtesy Leonid Zhigilei.

Zhigilei touts the value of XSEDE as a whole. “When we write our allocation requests, we often ask for time on different computers, and we also take advantage of other XSEDE resources, of course," he says. "We use VisIt software for visualizations. In addition, many of my students enjoy the file transfer service, Globus Online, which is very efficient in moving large files that we are generating in our simulations.”

During 2015, Zhigilei’s group published two papers in the journal Physical Review with different collaborators. Working with Henry Helvajian of The Aerospace Corporation, they discovered they could use surface acoustic waves (SAWs) to move tiny particles of gold on a silicon surface. In their article, they note that the use of SAWs has broad implications for applications in which heating must be avoided.

Collaboration with the experimental group of Peter Balling of Aarhus University, Denmark, led to a publication showing that the volume of a metal on a surface could be increased, providing new opportunities for tailoring surface properties to the needs of practical applications.

Going forward, Zhigilei’s group will study how metal nanoparticles efficiently convert laser energy absorbed at a surface, a key capability to a growing number of imaging and therapeutic biomedical techniques, Wu says.

“For example, researchers have demonstrated that laser irradiation of gold or silver nanoparticles attached to gene markers and delivered to specific cells can be used for selective killing of cancer cells or bacteria,” Wu explains. “In the area of drug delivery, doping the walls of microcapsules with metal nanoparticles opens a way for the remote release of encapsulated material into specific cells by targeting metal nanoparticles with near-infrared laser irradiation.”

Yet another illustration of big implications for humanity found at the nanoscale.

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